1. Field of the Invention
The present invention relates to a muting control device capable of controlling an audio signal to be in a mute mode.
2. Description of the Related Art
In an audio equipment such as an amplifier device, a volume control device capable of controlling an input audio signal to be in a mute mode and attenuating an audio signal outputted to a subsequent amplifier circuit so that the volume level of the signal is set to zero. Examples of occasions to control the audio signal to be in the mute mode include an occasion where an instruction to control the signal to be in the mute mode is input through a user operation, where the amplifier device is switched to a power-on state from a stand-by state, and where a selector circuit that selects a single audio signal out of a plurality of input audio signals to output the selected signal to the amplifier circuit is switched (e.g., from a tuner to a CD player). The signal is muted when shifting to the power-on state or when switching the selector circuit in order to reduce the pop noise level, thereby suppressing a pop noise from being reproduced through a loud speaker.
As one method of controlling an audio signal to be in the mute mode, typically, a muting circuit is provided in a signal line of the audio signal. The muting circuit is constituted from resistors inserted in series in an audio signal line and a transistor inserted between the audio signal line and the ground potential. Turning on the transistor by a control signal from a microcomputer connects the audio signal line to the ground potential, thereby attenuating the level of the audio signal outputted to the subsequent amplifier circuit. The amount of attenuation of the audio signal when controlling the muting circuit to be in the mute mode is determined based on the resistance values of the inserted resistors, and in order to increase the attenuation amount of the audio signal so that the volume level of the signal is completely set to zero, the resistance values must be increased. Alternatively, a combined resistance must be increased by providing a number of muting circuits in series, each constituted from the resistors and the transistor. However, increasing the resistance value or providing a number of muting circuits in series also provides a large impedance component that is unnecessary for the audio signal in the audio signal line, and thus can cause deterioration in the sound quality of the audio signal in a normal reproduction mode which is in an unmute mode.
Japanese Unexamined Patent Publication No. 06-120752 discloses a volume control device capable of controlling an audio signal to be in the mute mode by controlling to set the volume level of an electronic volume to zero. However, due to the nature of the electronic volume, it is difficult to control to set the volume level to zero instantaneously when an instruction to set the volume level to zero is input from a microcomputer, and a certain length of time period is required before the volume level is set to zero. Accordingly, a pop noise can be disadvantageously outputted through a loud speaker before the volume level of an electronic volume is set to zero in a case where an audio signal is controlled to be in the mute mode when an amplifier device is turned on from a stand-by state or when a selector circuit is switched.